fix(auth): improve user identifier retrieval (#2314)

* fix(auth): improve user identifier retrieval

* updates

Author: demolaf

app/src/main/java/com/firebaseui/android/demo/HighLevelApiDemoActivity.kt

@@ -42,6 +42,8 @@ import com.firebase.ui.auth.configuration.theme.AuthUITheme
 import com.firebase.ui.auth.ui.screens.AuthSuccessUiContext
 import com.firebase.ui.auth.ui.screens.FirebaseAuthScreen
 import com.firebase.ui.auth.util.EmailLinkConstants
+import com.firebase.ui.auth.util.displayIdentifier
+import com.firebase.ui.auth.util.getDisplayEmail
 import com.google.firebase.auth.actionCodeSettings
 
 class HighLevelApiDemoActivity : ComponentActivity() {
@@ -211,7 +213,7 @@ private fun AppAuthenticatedContent(
     when (state) {
         is AuthState.Success -> {
             val user = uiContext.authUI.getCurrentUser()
-            val identifier = user?.email ?: user?.phoneNumber ?: user?.uid.orEmpty()
+            val identifier = user.displayIdentifier()
             Column(
                 modifier = Modifier.fillMaxSize(),
                 horizontalAlignment = Alignment.CenterHorizontally,
@@ -263,7 +265,7 @@ private fun AppAuthenticatedContent(
         }
 
         is AuthState.RequiresEmailVerification -> {
-            val email = uiContext.authUI.getCurrentUser()?.email ?: stringProvider.emailProvider
+            val email = uiContext.authUI.getCurrentUser().getDisplayEmail(stringProvider.emailProvider)
             Column(
                 modifier = Modifier.fillMaxSize(),
                 horizontalAlignment = Alignment.CenterHorizontally,

auth/src/main/java/com/firebase/ui/auth/ui/screens/FirebaseAuthScreen.kt

@@ -78,6 +78,8 @@ import com.firebase.ui.auth.ui.screens.email.EmailAuthScreen
 import com.firebase.ui.auth.ui.screens.phone.PhoneAuthScreen
 import com.firebase.ui.auth.util.EmailLinkPersistenceManager
 import com.firebase.ui.auth.util.SignInPreferenceManager
+import com.firebase.ui.auth.util.displayIdentifier
+import com.firebase.ui.auth.util.getDisplayEmail
 import com.google.firebase.auth.AuthCredential
 import com.google.firebase.auth.AuthResult
 import com.google.firebase.auth.MultiFactorResolver
@@ -733,7 +735,7 @@ private fun AuthSuccessContent(
     onManageMfa: () -> Unit,
 ) {
     val user = authUI.getCurrentUser()
-    val userIdentifier = user?.email ?: user?.phoneNumber ?: user?.uid.orEmpty()
+    val userIdentifier = user.displayIdentifier()
     Column(
         modifier = Modifier.fillMaxSize(),
         verticalArrangement = Arrangement.Center,
@@ -783,7 +785,7 @@ private fun EmailVerificationContent(
     onSignOut: () -> Unit,
 ) {
     val user = authUI.getCurrentUser()
-    val emailLabel = user?.email ?: stringProvider.emailProvider
+    val emailLabel = user.getDisplayEmail(stringProvider.emailProvider)
     Column(
         modifier = Modifier.fillMaxSize(),
         verticalArrangement = Arrangement.Center,

auth/src/main/java/com/firebase/ui/auth/util/UserUtils.kt

@@ -0,0 +1,38 @@
+/*
+ * Copyright 2025 Google Inc. All Rights Reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
+ * in compliance with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software distributed under the
+ * License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either
+ * express or implied. See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.firebase.ui.auth.util
+
+import com.google.firebase.auth.FirebaseUser
+
+/**
+ * Returns the best available display identifier for the user, trying each field in order:
+ * email → phoneNumber → displayName → uid.
+ *
+ * Each field is checked for blank (not just null) so that an empty string returned by the
+ * Firebase SDK falls through to the next candidate rather than being displayed as-is.
+ * Returns an empty string if the user is null.
+ */
+fun FirebaseUser?.displayIdentifier(): String =
+    this?.email?.takeIf { it.isNotBlank() }
+        ?: this?.phoneNumber?.takeIf { it.isNotBlank() }
+        ?: this?.displayName?.takeIf { it.isNotBlank() }
+        ?: this?.uid
+        ?: ""
+
+/**
+ * Returns the user's email if it is non-blank, otherwise returns the provided [fallback].
+ */
+fun FirebaseUser?.getDisplayEmail(fallback: String): String =
+    this?.email?.takeIf { it.isNotBlank() } ?: fallback